1. Field of the Invention
The present invention relates generally to waveguide reflectors and specifically to a reflector for use in the resonating cavity portion of a free electron laser (FEL).
2. Description of Related Art
Two-stage free electron lasers typically utilize magnetic mirrors at the opposite ends of their resonating cavities to reflect the pump field along the optical axis of the cavity. A typical example of a free electron laser (FEL) is illustrated in U.S. Pat. No. 4,438,513 issued in the name of Luis Elias and assigned to the United States of America.
An FEL operated in the microwave to far-infrared bands, however, requires a high-Q resonator through which an electron beam must be passed. Since future FELs must be capable of generating very high average power, the resonators therein will have to be overmoded, i.e., use quasi-optical propagation with cross sections larger than a half wavelength to avoid thermal damage to the resonator structure. The expanding Gaussianmode patterns of a conventional confocal or a concentric resonator, however, do not easily fit within the bore of the wiggler-magnet array, especially at frequencies below 100 GHz. As a result, the radiation fields of such conventional resonators must either be confined by some means along the 30-100 cm interaction length or the resonator must be very large, on the order of 10 meters.
Reflectors utilizing square-wave corrugations operating with the known principle of Bragg scattering relating to constructive interference at certain angles called Bragg angles, have been previously proposed for use in waveguides. See, e.g., "Waveguide Resonators with Distributed Bragg Reflectors" by R. Kowarschik and A. Zimmerman, Optica Acta 1982, Vol. 29, No. 4, pages 455-462. According to the prior art literature, however, the shape of the corrugations has little effect on the reflection coefficient. See, e.g., articles by: Marcuse, IEEE Journal of Quantum Electronics, Vol. QE8, pages 661-669, July 1972; and Miles and Grow, IEEE Journal of Quantum Electronics, Vol. QE14, No. 4, pages 275-282 April 1978; and Yariv, et al., IEEE Journal of Quantum Electronics, Vol. QE13, pages 233-251, April 1977.
Couplers using diffraction gratings built directly into a dielectric waveguide for use in input or output coupling from the waveguide modes to free-space, or substrate propagating modes, are discussed by Yariv, et al., IEEE Journal of Quantum Electronics, Vol. QE 13, pages 233-251, April 1977 at pp. 249-251. The waveguide grating couplers discussed by Yariv et al. are built not into metallic waveguides but into dielectric waveguides, and their function is to scatter power out of the waveguide rather than coherently reflecting it inside the waveguide. Also, the corrugations are not blazed. The discussion includes an analysis of the coupling loss of such a waveguide grating with sawtooth-type triangular corrugations.
Bratman et al, in "FELs with Bragg Reflection Resonators Cyclotron Autoresonance Masers Versus Ubitrons", IEEE Journal of Quantum Electronics, Vol. QE19, No. 3, pages 282-95, March 1983, discuss the applications of sinusoidal corrugated reflectors in FELs, Ubitrons and Cyclotron Autoresonance Masers (CARM).
The sawtooth type triangular corrugations shown in FIG. 35 of the Yariv article and the sinusoidal corrugations of Bratman et al. are not spaced apart with an intervening base.
It is known in the field of optics, particularly as related to diffraction gratings, that "blazing" of grooves of a grating will cause it to be particularly reflective of light at a certain wavelength. See, e.g., Fundamentals of Optics, Jenkins and White, McGraw-Hill, 1957. As defined in the McGraw-Hill Dictionary of Scientific and Technical Terms, 3rd Edition, a "blaze-of-grating technique" is an optics technique whereby ruled grooves of a diffraction grating are given a controlled shape such that they reflect as much as 80% of the incoming light into one particular order for a given wavelength.
The understanding and applicability of blazing techniques, however, have previously been limited to the optics field and no prior attempts have been made to apply such techniques to waveguides or to free electron lasers.
Accordingly, it is the principal object of the present invention to coherently reflect power at specified wavelengths with a blazed corrugated reflector.
It is another object of the present invention to eliminate the use of reflective mirrors in FELs, and allow unobstructed passage of the electron beam.
A further object of the invention is to allow the generation of high power in an FEL.